The demand for eco-friendly vehicles is increasing due to the constant demand for fuel efficiency improvement for vehicles and the strengthening of exhaust gas regulations in many countries. As a practical alternative to this, a hybrid electric vehicle/plug-in hybrid electric vehicle (HEV/PHEV) is provided.
Such a hybrid vehicle can provide optimal output and torque depending on how the engine and motor are operated in harmony in the course of driving with the two power sources. Particularly, in a hybrid vehicle adopting a parallel type hybrid system in which an electric motor and an engine clutch (EC) are mounted between the engine and the transmission, the output of the engine and the motor can be simultaneously transmitted to a drive shaft.
Generally, in a hybrid vehicle, electric energy is used during initial acceleration (i.e., EV mode). However, since electric energy alone has a limitation in meeting the driver's required power, use of the engine as the main power source is eventually required (i.e., the HEV mode). In such a case, in the hybrid vehicle, when the difference between the number of revolutions of the motor and the number of revolutions of the engine is within a predetermined range, the engine clutch is engaged so that the motor and the engine rotate together. At this time, when the number of revolutions is too low, engine stall may occur when the engine clutch is engaged. Therefore, the hybrid vehicle controls the number of revolutions of the engine and the motor such that the engine clutch starts to be engaged at a specific revolution number (hereinafter, referred to as “target engagement speed” for convenience) set in a relatively safe zone in the stall of the engine. The target engagement speed can be set differently according to the engine characteristics of the vehicle or the gear stage of the engagement time.
However, when the engine is started immediately when it is needed as a main power source, a delay often occurs until the engine clutch is actually engaged and the force of the engine is transmitted to the axle of the drive wheel. As a result, fuel loss occurs while the power of the engine does not contribute to driving, which is called “non-driving fuel loss”. The manner in which a non-driving fuel loss occurs will be described with reference to FIG. 1.
FIG. 1 is a view for explaining an example of a form in which a non-driving fuel loss occurs in a general hybrid vehicle.
Referring to FIG. 1, when a driver operates the accelerator pedal (i.e., APS on), the required torque becomes large, and when it is determined by the vehicle that the driving power of the engine is required, the engine is started.
Since the engine is not loaded at the time of engine start-up, engine Speed (EngSpeed) rises rapidly but the motor speed (MotSpeed) may not reach the target engagement speed. In this case, the engine remains idle at the target engagement speed up to the engagement time, during which time a non-driving fuel loss occurs.
The above-mentioned non-driving fuel loss problem becomes more problematic when the target engagement speed is switched due to the gear shift. This will be described with reference to FIG. 2.
FIG. 2 is a view for explaining an example of a mode in which a non-driving fuel loss occurs in a general hybrid vehicle due to a gear shift.
Referring to FIG. 2, when the driver operates the accelerator pedal (i.e., APS on), the required torque becomes large, and when it is determined by the vehicle that the driving power of the engine is required, the engine is started. However, if a gear shift occurs before the target engagement speed is reached, not only the target engagement speed varies, but also the engagement of the engine clutch is delayed from the gear shift start time point to the gear shift end time point.
As a result, after the engine is started, when a gear shift occurs before the target engagement speed is reached, there is a problem that the non-driving fuel loss is additionally generated as much as the gear shift interval.